Run web benchmarks at -O2 to evaluate performance. (#162625)

As per flutter/flutter#162620, we are going to
run the web benchmarks at `-O2` for a trial period to evaluate the
performance difference, which will give us some data on whether we can
consider changing to `-O2` by default.

Author: eyebrowsoffire

dev/devicelab/lib/tasks/web_benchmarks.dart

@@ -38,7 +38,8 @@ Future<TaskResult> runWebBenchmark(WebBenchmarkOptions benchmarkOptions) async {
       options: <String>[
         'web',
         '--no-tree-shake-icons', // local engine builds are frequently out of sync with the Dart Kernel version
-        if (benchmarkOptions.useWasm) ...<String>['-O4', '--wasm', '--no-strip-wasm'],
+        '-O2',
+        if (benchmarkOptions.useWasm) ...<String>['--wasm', '--no-strip-wasm'],
         '--dart-define=FLUTTER_WEB_ENABLE_PROFILING=true',
         '--profile',
         '--no-web-resources-cdn',